Control system



Sept. 11, 1945. s. 5. ISSERSTEDT CONTROL SYSTEM- Filed March 26, 1943 4 Sheets-Sheet l 4 Sheets-Sheet 2 s. G. ISSERSTEDT CONTROL SYSTEM I Filed March 26, 1943 Sept. 11, 1945.

Sept. 11, 1945. s. G. ISSERSTEDT CONTROL SYSTEM Filed March 26, v 1943 4 Sheets-Sheet 4 f 0 Y .m H 5 w v y m m M F1 m m K w MW Sr Patented Sept. 11, 1.945

CONTROL SYSTEM Siegfried G. Isserstedt, Toronto, Ontario, Canada, assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application March 26, 1943, Serial No. 480,667 13 Claims. (Cl. 172-239) system in which the position of the controlled member may be varied in a very large number relatively small finite steps.

Another objectis to provide an improved selfbalancing remote position control system of the type in which the power available to position the controlled member is not dependent upon either the power supplied by the controlling member or the power applied to the controlled member.

Another object of the present invention is to provide an improved self-balancing remote position control system in which the movable controlling and following members are continuously rotatable.

Another object is to provide, in a remote position control system, an improveditransmitter and receiver unit, comprising an elongated'impedance element curved to form a substantially complete circle having a gap at the terminals of the element, sliders for cooperating with said element, and means for making the receiver slider jump the gap between the terminals of its impedance element when the transmitter slider moves through a corresponding range of positions.

A still further object is to provide, in such transmitter and receiver units, a variable im pedance comprisin an elongated resistance ele ment with tap connections at spaced points thereon and a series of spaced arcuate contact segments connected in sequenceto said taps and forming a substantially complete circle, so that the impedance is adjusted in a series of finite steps as a slider moves over the segments.

Other objects and advantages of my invention will become apparent from a consideration of the appended claims, specifications and drawings, inwhich Figure 1 represents, somewhat diagrammatically, a remote position control system including a simplified embodiment of my invention,

Figure 2 represents, somewhat diagrammatically, a portionoi a remote position control system including a preferred embodiment of my invention,

Figures 2 and 3.

Referring now to Figure 1, there is shown a transmitter unit Ill and a receiver unit 20, which may be located remotely from the transmitter. The transmitter unit I0 includes an elongated resistance element ll, of substantially circular form, having a pair of terminals l2 and iii. The resistance element ll extends almost completely around a circle about a shaft It as a center, with only a relatively small gap between the terminals I 2 and I3. The transmitter I0 is also provided with two pairs of semi-circular contact segments l5, l6 and l1, l8. These contact segments are mounted concentrically with the resistance element H. The segments l5 and I6 are of equal radius, and the segments i1 and I8 are of equal radius. The ends of both sets of segments of equal radius are spaced apart, and the spaces between the ends of the segments are aligned with the space between terminals l2 and I3 of resistance element H, or are diametrically spaced therefrom. I

The shaft I4 may be rotated by any desired device whose motion is to be transmitted to the receiver unit 20, as for example, a hand crank 2|. Fixed on the shaft It so as to be rotated therewith is a two-part slider arm, l9-22. Supported at one end of the arm 22 is a slider 23 adapted to engage the resistance element ll. Thisslider 23 is not quite long enough to bridge the gap between terminals 12 and I3. Supported at the end of the arm 19 are a pair of brushes 24 and 25. Brush 2! is adapted to engage the contact segments I5 and i6, while the brush 25 is adapted to engage the contact segments l1 and i8. Brushes 24 and 25 are slightly shorter than the gaps between contact segments l5 and i6 and I1 and I8 respectively. The two parts of the slider arm I922 are electrically insulated by an insert 26 of insulating material.

The transmitter I0 is also provided with a circular contact ring 21, concentric with shaft l4, and the arm 22 which carries the slider 23 also carries a brush 28 cooperating with the contact ring 21.

Five conductors, numbered 30, 3|, 32, 33 and 34, connect the transmitter ill with the receiver 20. At. the transmitter, conductor 30 is connected to, terminal I! of resistance element II, and to contact segment l5. Conductor 3| is connected to contact segment ll, conductor 32 is connected to contact ring 21, conductor 33 is connected to contact segment l5, and conductor 34 is connected to contact segment i5 and to terminal 13 of resistance element I i.

The receiver unit 20 includes an elongated resistance element 35, having terminals 35 and 31. The resistance element 35 is arcuate, forming a circle which is substantially complete except for a small gap between the terminals 35 and 31. At the center of the circle formed by the resistance element 35 is a shaft 40. Concentric with the resistance element 35 about the shaft 45, are two pairs of substantially semi-circular contact segments 41, 42 and 43, 44. The segments 41 and 42 are of equal radius, and the segments 43 and 44 are of an equal radius different from that of segments 4| and 42. A contact ring 45 is also concentric with the shaft 40.

Fixed on the shaft 45 so as to rotate therewith is a two-part slider arm 39-45. Am 45 carries a slider 41 cooperating with the resistance element 35 and a brush 43 cooperating with the contact ring 45. Slider 41 is suiliciently long to completely bridge the gap between terminals 35 and 31. The other slider arm 33 carries a brush 5n cooperating with the contact segments 4|, 42, and a brush 5i cooperating with the contact segments 43, 44. An insert 52 of insulating material electrically separates the two parts of the slider arm 39-45.

The brush 5!! is sufficiently long to bridge the gap between the lower ends of segments 4| and 42, but is shorter than the gap between the upper ends of such segments. Similarly, the brush 5| is sufliciently long to bridge the gap between the lower ends of segments 43 and 44, but is shorter than the gap between the upper ends thereof.

The shaft 40 is rotated by a, reversible motor 53 acting through a gear train schematically indicated at 54. Motor 53 is shown as being of the split-phase type, and has a pair of field windings 55 and 56 which are separated 90 electrical degrees in space. Power is supplied to motor 53 from a transformer 55 having a primary winding 61 and a secondary winding 52. The direction of rotation of motor 53 is controlled by a balanced relay 53,

The relay 63, which may be of the type disclosed in the patent to Willis H. Gille, No. 2,169,- 141, dated August 8, 1939, has a pair of opposed windings 64 and 55 which control the position of a balanced armature 55, pivoted at a point 51. Armature 65 carries a switch arm which is selectively engageable with either of a pair of stationary contacts 11 and 12, depending p n the position of armature 55 with respect to its pivot 51.

A pair of fixed resistance elements 13 and 14 are shown as being included in the receiver imit 20, although they might with equal facility be included in the transmitter unit II.

The transmitter 15 and the receiver 25 may be supplied from any suitable source of direct or alternating electrical energy. By way of example,

there is shown a transformer 15 having a primary winding 15 and a secondary winding 11. Both primary winding 15 of transformer 15 and primary winding 51 of transformer 55 may be connected to any suitable source of alternating electrical energy. such as supply lines 55 and II Resistance element ll of transmitter unit 15 and resistance element 35 of receiver unit 25,

together with their circuit connections, form a. balanced electrical network, which may be termed a kind of bridge circuit. This balanced network includes a, pair of branches connected in parallel to the secondary winding 11 of transformer 15. Each of these parallel branches includes one of the windings 54 and 55 of the balanced relay 53. When the current in the two branches is equal, the armature of relay 53 is at its central position, in which switch arm 15 engages neither of the contacts 11 and 12. When the current in either winding 54 or winding 55 is greater than that in the other winding, armature 55 is rotated about its pivot 81, thereby causing switch arm 10 to engage either contact 1| or contact 12.

One of the parallel branches of the balanced network may be traced from the upper terminal of transformer secondary winding 11 through conductor 32, contact ring 21, brush 23, slider arm 22, slider 23, the part of resistance element H between slider 23 and terminal l2, terminal l2, conductor 35, resistor 13, a conductor 52, relay winding 54, a conductor 53, terminal 35 of resistance element 35, that part of element 35 between terminal 35 and slider 41, slider 41, slider arm 45, brush 48, contact ring 45, and a conductor 54 to the lower terminal of transformer secondary winding 11.

The other parallel branch of the balanced network may be traced from the upper terminal of transformer secondary winding 11 through conductor 32, contact ring 21, brush 25, slider arm 22, slider 23, that part of resistance element ll between slider 23 and terminal 13, terminal i3, conductor 34, resistance element 14, a conductor 35, relay winding 55, a conductor 55, terminal 31. of resistance element 35, that part of resistance element 35 between terminal 31 and slider 41, slider 41, slider arm 45, brush 45, contact ring 45 and conductor 34 to the lower terminal of transformer secondary winding 11.

The resistance of element I l is preferably equal to that of resistance element 35. The resistance of elements 13 and 14 is somewhat greater than twice the resistance of either element II or element 35. The reason for this relationship between the resistances of the various elements will become apparent from a consideration of the operation of this system.

Operation 0/ Figure 1 From the preceding description, it should be apparent that the Junctions of the two parallel branches in the balanced network are the sliders 23 and 41. When the slider arms 22 and 45 are at corresponding angular positions, as shown in the drawings, the resistances in the two parallel branches between the sliders 23 and 41 are equal. For example, in the position shown in the drawings, the first parallel branch includes three re-, sistances; namely, the small portion of resistance element ll between slider 23 and terminal l2, resistance 13, and the large portion of resistance element 35 between terminal 35 and slider 41. On the other hand, the second parallel branch also includes three resistances; namely, the large portiim of resistance element 1 i between slider 23 and terminal 13, resistance 14, and-the small portion of resistance element 35 between terminal 31 and slider 41. The resistance of the small portion of element II is equal to the resistance of the small portion of element 35, and the resistance of the large portion of element H is equal to the resistance of the large portion of element 35. Furthermore, resistance 13 equals resistance 14. Therefore, the resistances of the two paths are equal, and the currents flowing in the.

two parallel branches are also equal. windings 64 and 65 of relay 66 are equally energized and the armature 66 is maintained at its center or neutral position, and the switch arm 16 engages neither contact 1| nor contact 12.

Under these conditions,-let it be assumed that the slider arm 22 moves a short distance counterclockwise, thereby moving slider 23 towards the terminal |2 of resistance element When such a movement takes place, the resistance in the first parallel branch of the balanced circuit is decreased, while the resistance in the second parg'agement with stationary contact 1|.

Engagement of switch arm with contact 1| completes an energizing circuit for the windings of motor 53. The circuit for winding 56 may be r traced from the upper terminal of transformer secondary winding 62, through a conductor 90, switch arm 10, contact 1|, a conductor 9|, winding 56, and a conductor 92 to the lower terminal of transformer secondary winding 62. The'energizing circuit for winding 55 may be traced from the upper terminal of transformer secondary winding 62'through conductor 90, switch arm 10,

contact, conductor 9|, a condenser 93, a conductor 94, winding 55, and conductor 92 to the lower terminal of transformer secondary winding 62. The energizing current flowing through winding 55 is displaced in phase approximately 90 electrical degrees by the condenser 93, from the current flowing through windin 56. In accordance with the well known characteristics of split-phase condenser motors, this diiference in phase between the currents in windings 55 and 56 causes rotation of motor 53 in a predeterm 'ned direction. In this case, the direction is such that the gear train '54 rotates shaft 46 counter-clockwise, thereby driving slider arm 46 toward an angular position corresponding to that assumed by slider arm This movement of slider arm 45 increases the resistance in the first parallel branch of the balancednetwork, and decreases the resistance in the second branch, thereby tending to rebalance the network. When the slider arm 46 reaches an angular position corresponding to that of the new position of slider arm 22, the network is again balanced, and the armature 66' moves back to its center position, thereby de-energizing motor 53. and stopping the movement of slider arm 46.

On the other hand, when the slider arm 22 moves clockwise from the position shown, the resistance in the first parallel branch of the balanced circuit is increased, while the resistance in the second branch is decreased. The current flowing through winding 64 of relay 6! is accordingly decreased, while the current in windin 65 is increased. Armature 66 therefore rotates counter-clockwise, moving switch arm 16 into engagement with contact 12.

Engagement of switch arm 10 with contact 12 completes energizing circuits for the windings 55 and 56 of motor 52. The circuit for winding 55 may be traced from the upper end of transformer 'secondary winding 62, through conductor 66,

switch arm 16, contact 12, a conductor 65, conductor l4, winding 55 and conductor 82 to the lower end of secondary winding 62. The circuit 101- winding 56 may be traced from the upper end of secondary winding 62, through conductor 60, switch arm 15, contact 12, conductor 95, condenser 93, winding 56, and conductor 62 to the lower end of secondary winding 62.

Condenser 63 is now in series with winding 56, while winding is connected to the power supply directly. The phase relations of the currents in the windings '55 and 56 are therefore reversed with respect to the phase relations existing under the conditions previously described, and the motor rotates in the opposite direction, turning shaft 46 clockwise so as to bring slider arm 46 into angular agreement with slider arm 22.

Now let it be assumed that the slider arm 22 is continuously moved counter-clockwise from the position shown in the drawings. The slider arm 46 is driven by motor 53 to follow this motion until the slider 22 moves out of engagement with terminal l2 of slider At that point, the circuits through both parallel branches of the balanced network are broken, since the slider 23 is slightly narrower than the gap between terminals l2 and i5. Both windings 64 and 65 of relay 53 are therefore de-energized,- and motion of slider 46 is stopped with slider 41 in engagement with terminal 31.

When slider 23 moves across the gap and engages terminal II, the resistance of the first parallel branch of the balanced network is greatly increased, since it includes all of both resistance elements II and 55. The second parallel branch, on the other hand, includes only the resistance 14. It means were not provided to prevent it, this would greatly reduce the current through winding 64 of relay 63, and greatly increase the current through winding I55. Switch arm 10 would then engage contact 12, thereby energizing motor 53 for operation in a sense to drive the slider arm 46 clockwise. This would cause slider arm 46 to move in a direction opposite to that in which slider arm 22 was moving, thereby destroying the whole effect of the remote position a connection shunting the resistance element 13 in the first parallel branch of the balanced network. This shunt connection may be traced from the upper terminal of resistance 13 through conductor 30, contact segment l5, brush 24, slider arm l6, brush 25, contact segment I1, conductor 5|, contact segment 43, brush 5|, slider arm 39, brush 50, contact segment 4|, and a conductor 66 to the lower terminal 01' resistance element 15. The first parallel branch of the balanced network then includes only the resistance elements II and 65, while the second parallel branch includes only the resistance element 14. Since the'resistance oi element 14 is somewhat greater than twice, that of element II, which is equal to that of element 65, it will be seen that the resistance in the second parallel branch is greater than that in the first parallel branch. Therefore, the current in the first parallel branch is greatest, and relay winding 64 is energized more highly than relay winding 65. Contact arm 16 is thereby moved intoengagement with contact 1|, causing motor 53 to be energized for operation in a direction to drive slider arm 46 counter! clockwise, thereby moving'it across the gap be-,.

corresponding .with the position of slider arm 22.

In a similar manner, it the slider 24 crosses the gap in a clockwise direction,- while the slider 41 is on the left side of its associated gap, opposite the position shown in the drawings, a connection is completed shunting the resistance element 14. This connection may be traced from the upper terminal of resistance element 14, through conductor 34, contact segment I4, brush 24, slider arm l9, brush 25, contact segment I8, conductor 33, contact segment 44, brush 5|, slider arm 39, brush 50, contact segment 42, and a conductor 41 to the lower terminal of resistance element 14. Shunting 01' resistance 14 reduces the resistance of the second parallel branch of the balanced network below that of the first parallel branch, thereby causing energization of relay winding 65 to be greater than that of winding 64. Switch arm 14 is thereby operated into engagement with contact 12, causing motor 53 to rotate in a direction to drive slider 46 in a clockwise direction across its gap.

As stated above, the slider 23 and the brushes 24 and 25 of transmitter III are narrower than the gaps in their associated contact segments and in resistance II, while in the receiver 24 the slider 41 and the brushes 50 and il are wider than the gaps in their associated resistor and the lower ends of the contact segments. If it were possible for'the slider 24 to shunt the gap in the resistance element II, the position of slider 41 would control the relay 43 at such times, and the motor 53 would be operated in a direction to move slider 45 towards its center position,

meaning the position at the center of slide wire 45. Such an effect is, of course, undesirable, as it is intended that the slider arm 44 follow the slider 42 as closely as possible at all times. Also, if the [brushes 24 and 24 were wider than their respective gaps, it would be possible to shunt both of the resistors 13 and 14 simultaneously. If this occurred, there might possiblyi'ollow a condition in which one of the relay windings 44 and 44 was connected directly across the transiormer terminals without any resistance in series therewith. Such an occurrence might of course cause damage to the relay and possibly to other equipment. Therefore, the slider 2-4 and the brushes 24 and 25 are made narrower than their respective gaps.

On the other hand, it is necessary that the slider arm 46 be able to start up again if it should stop in the middle of its associated Bap. If the slider 41 were narrower than the gap in slide wire 35, both the circuits to relay windings 44 and BI would be broken when the slider arm 44 stopped with the slider 41 in the center of the gap, and the receiver would thereafter remain stationary, as no movement of the transmitter could possibly cause energization of the relay 44. Therefore, the slider 41 must be made wider than the gap in the slide wire 44. Bimilarly,'the brushes 44 and 4| should be made wider than the gaps in which they stop when the slider 41. is at the center of the gap in slide wire 44, in order that, when starting up from that position, they may start in the proper direction. For example, when both slider arm 22 and slider arm 44 are in the centers of their associated slidewire gaps, either resistance 14 or 14 may be shunted upon movement of slider 24 in one direction or the other, thereby insuring a responsive movement of slider arm 44 in the proper disection to follow slider arm 22.

Figures 2,,3 and 4 I have shown in Figure 2 a modification of the system shown in Figure 1, in which the relay means controlling the receiver motor is an electronic amplifier, rather than the more conventional relay used in the case.of Figure 1. In the system of Figure 2, a diflerent type of balanced electrical network is used, which produces a voltage proportional to its unbalance, rather than the balanced network of Figure l, which produces a pair of currents, whose difference is the measure of the unbalance of the network. Also, in the system shown in Figure 2, the variable resistance elements of the transmitter and receiver are tappedat spaced intervals, and the taps are connected to arcuate contact segments arranged in the form of a circle. By the use of this type of variable resistance and contact struc ture, I have provided a circuit in which a predetermined value of unbalance voltage is produced when the network is unbalanced by a given amount, and in which the unbalance voltage of the network is varied in a series of substantially equal finite steps.

Slide-wires, such as those shown in Figure 1,

' wear unevenly, and after a long period of use the surface of the slide-wire is in eil'ect a series. of contact surfaces unevenly spaced along the length of the resistance. This uneven wear of the slide-wire produces irregular unbalancing effects on the network, depending upon the position of the transmitter and receiver sliders. Buch irregular response of the network is prevented by the use of the tapped resistor and arcuate contact segments shown in Figure 2.

In Figure 2 are shown a transmitter I44 and a receiver I 44. In the transmitter I44 is an elongated resistance element III, which is shown for convenience as being arranged substantially in the form of a circle, leaving only a small gap between its terminals I42 and I44. The resistance III is tapped at equally spaced intervals, as shown at I44, and each of the tape I44 is connected to an arcuate contact segment I44. The segments I44 are arranged in the form of a circle about a shaft I 44.

Two of the arcuate segments, numbered I41 and I44, are connected respectively to the terminals I42 and I 44 of the resistance element III. The segments I41 and 144 are hereinafter referred to as terminal segments. For the sake of clarity in describing the structure and operation of my invention, the terminal segments I41 and I44 are shown as more widely separated than the segments I44. It should be understood, however, that this wider spacing is not necessary, but that uniform spacing may be used between all aments.

Arranged concentrically about the shalt I44 are three rows of contact segments and a contact ring I24. The first, or outer, row comprises the lesments I44; the second comprises a pair of semi-circular segments H4 and III, and the third row includes two semi-circular groups of segments. The ends of the semi-circular segments Illand III are spaced from each-other, this space being displaced by an angle of from the space between the terminal segments I41 and I44. lachofthegrounsinthethlrdrowoonsists of a long segment and three short segments. The longsegments arenumbered H2 and!" in the drawlnls. while the short segments of each group are numbered 4 and Ill, respectively. Assodatedwithtbelegmentllllandlflisaredstwith the segments H and III.

bridge circuit has input terminals I50 and IN,-

ance element II5, hereinafter referred to as a compensating resistance, which is tapped at spaced intervals, each tap being connected to one of the segments H2 and H4. Similarly, a compensating resistance element H1 is associated with the segments I I3 and I I5, the resistance element II1 being tapped at spaced intervals and each tap being connected to one of the segments.

Fixed on the shaft I05 so as to rotate therewith is an arm I 2I of electrically insulating material, carrying a pair of diametrically opposite extensions III and H5, of electrically conductive material. Extension I I8 carries a pair of brushes I24 and I25, and extension II5 carries a pair of brushes I22 and I23. The brush I22 cooperates The brush I23 cooperates with the third row of contact. seements, including the segments H2, H3, H4 and II5. Brush I24 cooperates with contact ring I and brush I25 cooperates with the outer row of contact segments I05.

transmitter I00 and extend to the receiver I35.

Conductor I21 is attached to contact segment I I2. Conductor I28 is connected to terminal I02 of resistor I M Conductor I23 is connected to contact ring I20. Conductor I30 is connected to terminal I03 of resistor IOI. Conductor I3I is connected to contact segment I I3.

Segment III is not in electrical connection with any other part of the system, but merely provides a more uniform wearing surface for the brush I22. The receiver I comprises three rows of concentric contact segments and a contact ring I50. Each of the outer row of contact segments I36 is connected to one of a plurality of equally spaced taps I31 on a resistance element I38. The resistance element I38 is shown as forming a substantially complete circle, except for a small gap between its terminals I40 and, I4I. The terminals I40 and I H are connected respectively to two segments I42 and I43, hereinafter referred to as terminal segments.

The second row of contact segments comprises a pair of substantially semi-circular segments I44 and I45. The ends of the segments I44 and I45 are spaced from each other, and these spaces are aligned with the space between the terminal segments I42 and I43. The third row of contact segments comprises a pair of substantially semicircular segments I45 and I41. The segments I45 and I41 are also spaced from each other at their ends, and the spaces are aligned with the space between the terminal segments I42 and I43.

All of the segments and the contact ring I of the receiver I35 are concentric about a shaft I5I, which carries an arm I52 of electrically insulating material, carrying a pair of diametri cally opposite extensions I48 and I .45, of electrically conductive materiaL- Extension I48 carries a pair of brushes I53 and I54, and extension I 45 carries a pair of brushes Iand I55. Brush I53 cooperates with the semi-circular contact segments I44 and I45, brush I54 cooperates with segments I45 and I41, brush I55 cooperates with contact ring I50, and brush I55 cooperates with the outer row of arcuate segments I35.

The resistance element IOI of transmitter I00 and the resistance element I38 of receiver I35 are interconnected in a balanced electrical network of the Wheatstone bridge type, hereinafter referred to as the primary bridge circuit. This and the contact rings I20 and I50 serve as the output terminals. The customary four branches of a Wheatstone bridge circuit interconnect the input and output terminals.

The first branch of the bridge connects input terminal I50 and the contact ring I20. This branch may be traced from input terminal I60 through a conductor I52, a fixed resistance I53, conductor I30, terminal I03, a portion of resistance IOI, one of the taps I04, one of the contact segments I05, brush I25, extension II8 of arm I2I, and brush I24 to contact ring I20.

The second branch of the bridge circuit interconnects the input terminal I5I and contact ring I20. This branch may be traced from input terminal I5I through a conductor I54, a fixed resistance I55, conductor I28, terminal I02, a portion of resistance element I M, one of the taps I04, one of the contact segments I05, brush I25, extension II8 of arm I 2|, and brush I24 to contact ring I20.

The third branch of the bridge circuit may be traced from input terminal I50 through a conductor I55, a fixed resistance I51, a conductor I58, terminal I, a portion of resistance element I55, one of the taps I31,'one of the contact segments I35, brush I55, extension I45 of arm I52, and brush I55 to contact ring I50.

The fourth branch of the bridge circuit connects input terminal I5I with contact ring I50 and may be traced from terminal I6I through a conductor I10, a fixed resistance III, a conductor I12, terminal I40, a portion of resistance element I38, one of the taps I31, one of the segments I35, brush I55, extension I48 of arm I52, and brush I55 to contact ring I50.

Contact ring I 20 is connected through conductor I20 to an input terminal I13 of an electronic amplifier I 14. Contact ring I50 is connected through a conductor I15 to the other input terminal I15 of amplifier I14. 4

While any suitable electronic amplifier may be used, I prefer to use one of the type wherein the amplifier output is varied in direct relation to the magnitude of the input signal. A suitable amplifier is shown, for example, in the F. L. Moseley Patent No. 2,088,659, issued August 3, 1937.

In addition to the four main branches of the bridge circuit, the balanced network includes means effective under certain conditions to connect additional resistance in parallel with the first branch of the bridge circuit. This additional resistance comprises a fixed compensating resistance I19 and may also include all or a part of the compensating resistance II1. In order to illustrate the conditions under which various amounts of this additional resistance are connected in the circuit, consider that th receiver slider arm I52 is in a position such that brushes I53 and I54 engage segments I44 and I45, respectively, which may, for example, be the position shown in the drawings. Now consider that the transmitter slide'r arm I2I has been rotated clockwise from the position shown in the drawings, to a position wherein the brushes I25,

I22, and I 23 engage terminal segment I01, segment H0, and the left-hand one of the three segments II5, respectively. A connection may now be traced from bridge input terminal I50 through a conductor I11, resistance I15, a conductor I18, contact segment I45, brush I54, ex-

tension I48 of arm I52, brush I53, segment I44,

conductor III, segment II3, resistance II1, the left-hand one of the three segments II5, brush I23, extension IIQ of slider arm I2I, brush I22, segment H0, and conductor I25 to contact ring I20, which serves as an output terminal of the bridge circuit. It should be remembered that this connection includes resistance I10 and all of resistance III, and is in parallel with the first branch of the bridge circuit.

Now let it be assumed that the slider arm I2I is moved further in a clockwise direction, while the slider arm I 52 remains in a position such that brushes I53 and I54 engage segments I and I45, respectively. As brush I 25 moves into engagement with the next segment I05, brush I2l'moves into engagement with the center one of the three se ments H5, and a portion of the resistance III is removed from the connection Just traced. As brush I25 moves into engagement with the second segment I05 to the left oi. terminal segment I01, brush I23 moves into engagement with the righthand one of the three segments II5, thereby removing a second portion of resistance I" from the connection previously traced. As brush I25 moves into engagement with the third segment I05 to the left oi. terminal segment I01, brush I23 moves into engagement with segment I I3, thereby removing all of resistance II I from the connection which shunts the first branch of the bridge circuit.

It will be readily understood by those skilled in the art, in accordance with the well-known characteristics of Wheatstone bridge circuits, that the potential applied to the input-terminals of the bridge is divided in the same proportion between the two branches on either side of the bridge when the bridge is balanced. In other words, in the present bridge circuit, the potentials across the first and third branches are equal, and the potentials across the second and fourth branches are equal when the bridge is balanced. From the preceding description, it should'be further apparent that the bridge circuit is balanced when the transmitter and receiverbrushes I25 and I55 engage corresponding segments.

Now let it be assumed that the slider arms I2I and I52 are in corresponding angular positions,

so that the bridge is balanced, and that these positions are such that the segments engaged by the brushes I25 and I55 are substantially displaced from vthe respective terminal segments. Under these conditions, if the transmitter brush I25 moves over. one segment, in a clockwise direction, for example, a, potential appears at the bridge output terminals whose magnitude corresponds to the potential drop across the resistance between adjacent taps I04 on the resistance element II. If the transmitter brush I25 moves over more than one segment, the unbalance potential appearing at the bridge output terminals is proportional to the number of segments I05 corresponding to the diflerence in the angular positions of the transmitter and receiver slider arms. The polarity, or phase, of the unbalance potential depends upon the sense of displacement of the transmitter arm with respect to the receiver arm.

unbalance potential or the bridge circuit would have a magnitude and polarity corresponding to a counter-clockwise displacement or twenty-three segments between the transmitter arm and the receiver arm. The shunt connection for the first branch of the bridge circuit is completed at this time, however, and the resistances I10 and II I are so designed and proportioned with respect to resistances Ill and I35 that the resistance'oi the first branch is greatly decreased, and unbalance potential of the bridge circuit is substantially equal to the unbalance potential caused by a clockwise displacement of one segment at any other angular position of the transmitter and receiver sliders. Furthermore, if the transmitter slider arm I2I continues to move clockwise while receiver brush I55 remains in contact with terminal segment I, the amount of resistance removed from the first branch of the bridge by operation of slider I25 across segments I I5 may be so designed and proportioned with respect to the resistance inserted in the second armoi' the bridge by operation of brush I25 across segments I05, that the unbalance potential of the bridge remains proportional to the number of segments I05 corresponding to the diiiference in the angular positions of the transmitter and receiver sliders. Oi course, in the modification shown, this proportion is not maintained aiter the brush I23 engages segment II3, but it will be readily understood by those skilled in the art that it could be maintained as far as desired by increasing the length of resistor I I I and the number of segments II5.

A similar connection is provided for shunting the second branch of the bridge circuit when the transmitter slider arm I2I moves the slider I25 across the gap between terminal segments I01 and I05 in a counter-clockwise direction and the receiver brush I55 remains on the left-hand side of the gap betweenterminal segments I42 and I. In order to trace this second connection, I

assume that the transmitter slider arm I2I is in a position such that brushes I25, I22, and I23 engageterminal segment I08, segment I I 0, and the right-hand one of the three segments I I4, respectively. Further assume that the receiver slider arm I52 is in a position such that brushes I55 and I54 engage segments I45 and I", respectively. The connection for shunting the second branch of the bridge circuit may then be traced from input terminal I5I, through a conductor IOI, a compensating resistance I80, a conductor I82, contact segment I41, brush I54, extension I" of slider arm I52, brush I55, contact segment I45, conductor I21, contact segment II2, resistance Hi, the right-hand one of the three segments II, brush I25, extension II! o! slider arm I2I, brush I22, contact segment Ill, and conductor I20 to contact ring I20, which serves as an output terminal of the bridge circuit. The ei'iect of this connection shunting the second branch of the bridge circuit is entirely analogous to the eiiect of the connection shunting the first branch, and further explanation is believed to be unnec- Power is p lied to the balanced network from a secondary winding Ill of a transformer I04 having a primary winding Ill. The upper terminal oi secondary winding III is connected to input terminal I" through a conductor I05. The lower terminal of transformer secondary winding I 05 is connected to input terminal I5I through a conductor Ill.

Means are provided (or selectively connecting either-iinput terminal I50 or input terminal I of the bridge circuit directly to the output terminal represented by contact rin I20, thereby short-circuiting either the first or second branches of the bridge circuit. The connection for shortcircuiting the first branch of the bridge circuit may be traced from input terminal I60 through a conductor I90, a contact I9I, (see Figure 3), a switch arm I92, and a conductor I93 to conductor I29, which is connected to contact ring I20. The connection for shunting the second arm of the bridge circuit may be traced from input terminal IOI through conductors I81 and I94, a contact I95 (see Figure 3), a switch arm I95, and conductor I93 to conductor I29, which is connected to contact ring I20.

The switch arms I92 and I95 may be operated to engage their associated contacts I9I and I95 by means to be described later.

In order to simplify Figure 4 of the drawings, certain parts of this network, together with its power supply and the amplifier I14, are indicated therein as a unit I51. The contents of the unit may be ascertained by comparing Figure 3, which shows a dotted line I51 surrounding the elements which comprise the unit.

Amplifier I14 is supplied with energy from a secondary winding 200 of a transformer 20I having a primary winding 202. Primary windings I05 and 202 may both be connected to the same alternating current supply lines 2I5 and 2H.

Amplifier I14 has output terminals 203 and 204 connected through conductors 205 and 205, respectively, to the terminals of field windings 2H and 2I2 of a series motor 208. The opposite terminals of windings 2H and 2I2 are both connected to one terminal of armature 2I3 of motor The other terminal of armature 2 I3 is connected through a conductor 2 I 4 to one terminal of secondary winding 200.

The amplifier I14, as previously mentioned, is preferably one in which the amplifier output is varied in direct relation to the magnitude of the input signal. Furthermore, as described in the Moseley Patent No. 2,088,659, it has a characteristic such that the relative currents passing through output terminals 203 and 204 are differentially controlled in accordance with the phase of the input signal. In other words; when no signal appears at the input terminals I13 and I10, which condition occurs when the primary bridge is balanced, the current flowing through output terminal 203 and motor field winding 2 is substantially equal to the & current flowing through output terminal 204 and field winding 2I2. The construction of windings 2I I. and 2I2 is such. that the energization of one tends to cause rotation of the motor in a direction opposite to the direction of rotation caused by energization of the other. When the bridge is unbalanced, however, a signal appears at input terminals I13 and I18, and amplifier I14 responds to this signal in such a manner as to increase the current flowing in one of the windings 2I I or 2I2 and to decrease the current flowing through the other. The.

particular winding in which the current fiow is increased is determined by the phase of the input signal with respect to the phase of the terminal potential of transformer secondary 200. If the input signal is substantially in phase with this terminal potential, for example, the current fiow in one winding is greater, while if the input signal is opposite in phase, the current flow in the other winding is greater. It should therefore be apparent that upon unbalance of the bridge, the motor will be driven in one direction or the other, dependent upon the sense of unbalance of the bridge.

Motor 200 drives the shaft I5I through a gear train schematically indicated at 220'. The connections between the bridge, amplifier, I14, and

motor 200 are arranged so that upon unbalance of the bridge due to a change in angular position of the transmitter slider arm I2I, the motor 208 drives the receiver slider arm I52 in the proper direction toward a corresponding angular position, thereby rebalancing the bridge.

Since the amplifier output is varied in direct relation to the magnitude of the input signal, and the magnitude of the input signal is proportional to the difference between the angular positions of the transmitter and receiver sliders, it should be apparent that the motor speedv is substantially proportional to the angular displacement between the transmitter and receiver slider arms. This characteristic aids in obtaining a follow-up system in which the receiver follows the transmitter rapidly and without overshooting. For example, as the receiver approaches a balanced condition after an extreme unbalance, the energization of the motor is decreased, thereby permitting the motor to stop more quickly when the balanced condition is reached.

If desired, the bridge circuit could with equal facility utilize a direct current source of electrical energy in place of the transformer I84. If such a source were used, the amplifier I14 would respond to a difference in polarity of the unbalance potential in the same manner that it responds to a difference in phase of the alternating unbalance potential when an alternating source is used.

Figure 3 discloses a variable resistance device 225 forming a part of the transmitter unit I00, and another variable resistance device 226 forming a part of receiver unit I35. These variable resistance devices 225 and 225,are interconnected so as to form a second normally balanced electrical network connecting the transmitter and receiver units. This network is also of the Wheatstonebridge type, and is hereinafter referred to as the secondary bridge circuit.

The variable resistancedevice 225 includes a slide-wire resistance 230 and a contact ring 23I, both of substantially circular form and mounted concentrically with respect to a shaft 232. Fixed on the shaft 232 is a slider arm 233 carrying a slider 234 adapted to engage slide-wire 230 and a slider 235 adapted to engage the contact ring 23I. The contact ring 23I forms a, complete circle. The slide-wire230 forms a substantially complete circle. except for a small gap between its terminals 230 and 231.

The variable resistance device 225 of receiver I35 includes a slide-wire resistance 240 and contact ring 2, both of circular form and concentric with respect to a shaft 242. Fixed on the shaft 242 so as to rotate therewith is a slider arm 243 carrying a slider 244 which cooperates with slide-wire 240, and a slider 245 which cooperates with contact ring 2. The contact ring I forms a complete circle, and the slide-wire 240 forms a circle complete except for a small gap between its terminals 245 and 241.

A conductor 250 interconnects terminals 230 and 245, and a conductor 25I interconnects terminals 231 and 241. Contact ring 23I is connected by a conductor 252 to one input terminal 253 of an cmplifier 254. Contact ring 2 is connected by a conductor 255 to the other input terminal 255 of amplifier 254.

The amplifier 254 may be an electronic amplifier of any suitable type; for example, one of the type shown in Figure 2 of the copending application of Albert P. Upton, Serial No. 437,561, filed April 3, 1942.

Amplifier 254 is supplied with electrical energy from a secondary winding 251 of a transformer 258 having a primary winding 256. Primary winding 256 may conveniently be connected to supply lines 2I6 and 2I1.

Amplifier 254 has a common output terminal 260 and a pair of selectively energizable output terminals 26I and 262. A winding 266 of a relay 264 is connected between output terminals I and 260 by conductors 265 and 256, respectively. A winding 261 of a relay 266 is connected between terminals 260 and 262 by conductors 265 and 266 respectively.

Winding 263 of relay 264 is operatively associated with contact arm I96, so that when relay winding 263 is energized, switch arm I66 is moved into engagement with contact I65. Relay 266 includes switch arm I92, so that when winding 261 is energized, switch arm I62 is moved into engagement with contact I61.

It will be seen that the variable resistance devices 225 and 226 form a balanced electrical network of the Wheatstone bridge type. This is the secondary bridge circuit previously mentioned. Power is supplied to this network from a secondary winding 210 of a transformer 21! having a primary winding 212. The terminals of the transformer secondary winding 210 form the input terminals of the secondary bridge circuit, including the resistance devices 225 and 226. The contact rings 23I and 24I form the output terminal of this secondary bridge, which are connected to the input terminals of the amplifier. When the bridge is balanced, no signal is impressed on the input terminals of the amplifier, and neither of therelays 264 nor 266 are energized. When the network is unbalanced, one or the other relays 264. and 266 is energized, the particular relay to be energized being selected in accordance with the direction of unbalance of the bridge. as described in the copending Upton application previously referred to.

Referring now to Figure 4, it will be seen that transmitter I includes a pointer 215 which is rotatable with respect to a stationary scale 216. Pointer 215 is fixed on a shaft 211, which is connected through a gear train, schematically shown at 216, to shaft 232 of variable resistance device 225. Shaft 232 is connected through another gear train 260 to shaft I06 of variable resistance device IOI. Shaft I06 is connected through a third gear train 26I to any suitable means for driving pointer 215, which is shown, by way of example, as a hand crank 265.

In the receiver unit I35, the shaft ll of the variable resistance device I65 drives through a gear train 266 the shaft 242 of variable resistance device 226. Shaft 242 in turn drives through a gear train 261, a shaft 266 to which is attached a pointer 266 rotatable with respect to a stationary scale 26I.

Operation of Figures 2, 3, and 4 The ultimate function of the complete system, as disclosed in Figure 4, is to position receiver pointer 260 with respect to scale "I so that it corresponds with the position of transmitter pointer 215 with respect to scale 216. The transmitter pointer 215 may be moved with respect to scale 216 by rotation of the hand crank 265. The ratios of the gear trains 216, 266 and I are such that the variable resistance device IOI rotates more slowly than the crank 265, and shaft 232 of variable resistance device 225 rotates more slowly than shaft I06, and shaft 211 carrying the pointer 215 rotates more slowly than shaft 232. The particular gear ratios chosen are immaterial except that their magnitude determines the accuracy with which pointer 215 may be positioned with respect to scale 215. For example, in one embodiment of my invention, it was desired to position a pointer accurately to within %0 of a degree. In such a system, if the resistance device IOI has twenty-four contact segments, the gear trains 216 and 260 should be designed so that the shaft I06 completes one revolution every time the pointer 215 passes through one eight-hundredth of a circle. In other words, a gear ratio of 800 to 1 between shafts I06 and 211 is required.

The ratio of the gear train 260 is first chosen so that when shafts I66 and I5I are displaced from each other by an angle slightly less than 180 degrees, the secondary bridge circuit is unbalanced sufliciently to cause operation of one of the relays 264 and 266. The gear train 216 is then designed to give the desired accuracy of motion of pointer 215.

It will be readily understood that gear train 261 of receiver unit I55 is built with the same gear ratio as gear train 216 of transmitter I00, and that gear train 266 of the receiver has the same gear ratio as gear train 260 of the transmitter. There need be no particular relationship between the ratio of transmitter gear train MI and receiver gear train 220. Each of these ratios is separately determined from the amount of torque and speed available to operate the crank 265 and the torque and speed characteristics of the motor 206, respectively.

When the pointers 215 and 260 are in corresponding positions, both the primary and secondary bridge circuits are balanced, and no signal is applied to the input terminals of amplifier I14. Therefore, no power is supplied to winding 201 of motor 208, and the receiver pointer 260 remains stationary. Under these conditions, assume that hand crank 285 is rotated clockwise so as to turn pointer 215 clockwise with respect to its scale 216. This causes shaft I06 to turn counter-clockwise,

as indicated by the arrows in the drawings. Let it be assumed that this motion is sufllcient to move brush I25 from one of the segments I05 across the gap to the adjacent segment. This causes the primary bridge to be unbalanced by an amount corresponding to the voltage drop between two adjacent segments I05, and in a sense dependent on the direction of rotation of shaft I06. This unbalance voltage is applied to the input terminals of amplifier I14, thereby causing energization of motor 206 so as to rotate shaft I5I counterclockwise so as to follow the movement of shaft I06. As soon as shaft I5I has moved the brush I56 to a segment corresponding in angular position to that on which the brush I25 of transmitter I60 rests, the input signal at the amplifier I14 is again reduced to zero, and the motor 266 stops. By making the brushes I25 and I55 narrower than the segments I and I66, the motor is given a considerable amount of latitude as to its stopping position. In this way, the motor 266 may coast for several revolutions after de-energization, the number of revolutions depending on the design of gear train 220, without carrying the brush I55 on to the succeeding segment and thereby causing the system to'hunt.

\ grees.

9,884,629 The brushes should, however, be made wider than shafts I06 and III.

' If now the rotation of crank 205 continues until the slider I26 crosses the gap between the terminal segments I01 and I08, either the first or second branch of the primary bridge circuit will be shunted through one of the resistances H9 or I80, respectively, through the connections previously traced.

The resistance elements I19 and I80, and the resistance elements I I 6 and I'll may be so chosen and proportioned, as previously explained, that when the transmitter slider crosses the gap ahead 'of the receiver slider, the unbalance signal applied to the amplifier is exactly the same, per contact segment displacement between the two sliders, as when the sliders are moving along the central parts of their respective resistance elements III and I00.

The motor 208 is chosen such that its full running speed is greater than the maximum speed of crank 286, so that the transmitter, I00 can never get ahead of the receiver I65 by any appreciable amount. Nevertheless, the possibility remains that the transmitter shaft I06 might become displaced from the receiver shaft ISI by more than I80 degrees, and ifthis happened, the motor 208 might be reversed when the brush I crossed the gap, thereby causing the displacement between the transmitter and receiver pointers to increase. This might occur when the sysstem is first placed in operation, if the transmittcr is turned while the amplifier is first heating up.

The secondary bridge circuit including the variable resistance devices 225 and 226 is provided to prevent reversal of the motor when the shafts I06 and I5I are displaced by more than'180 de- The shafts 232 and 242, which operate the variable resistance devices 226 and 2-26, are geared down so that they rotate more slowly than the shafts I00 and lil. Since the drive shafts 222 and 242 rotate more slowly than the shafts I06 and IBI, the resistance devices 226 and 220 are less sensitive to changes in position of the pointers 216 and 290 than are the resistance devices III and I30. The gear trains 280 ments 2" and 240 are so chosen and related 'to the characteristics of amplifier 254, that the signal applied to the input terminals of amplifier 254 is not eflective to cause energization of either relay .264 or 268 until the displacement bethe slider arms 233 and 243 has reached an amount corresponding to a displacement slightly less than 180 degrees between shafts I06 and I-6I. When such a displacement occurs, one of the relays 264 or 268 is energized, depending upon the direction of displacement of the receiver with respect to the transmitter, thereby causing either the first or second arms of the primary bridge circuit to be shunted, connecting the bridge output terminal represented by the con- :tact ring I20 directly to either input terminal I or Ill Such a connection takes the variable resistance devicesll0l and I36 completely out of control 01 motor 200, which is then controlled only by the displacement between the variable resistance devices 226 and 220. It may therefore be seen that if the shaft III becomes displaced by more than 180.. from the angular position of the shaft I06, the variable resistance devices 226 and 226 will take control and operate motor 208 at its maximum speed until the position of pointer 290 again approaches correspondence )With that of pointer 215,

No provision has been made in the' systemdisclosed to prevent possible faulty operation of the system due to the sliders 234 and 244 gettingseveral revolutions before sliders 234 and 244 became separated sufficiently to lie on opposite sides of their respective resistance gap. Therefore, the possibility of the occurrence of such a condition is very remote. If in practice it became necessary to guard against such a condition, the secondary bridge circuit 251 could be provided with an arrangement for shunting a portion of it whenever the sliders became positioned on the opposite sides of their respective gaps, such as the arrangement provided for the primary bridge circuit.

While I have shown and described certain preferred embodiments of my invention, it will be readily understood that modifications thereof will readihr appear to those who are skilled in the art, and I therefore wish to be limited only by the scope of the appended claims.

I claim as my invention:

1. In a ibalancable electrical network, in combination, a control potentiometer and a rebalancing potentiometer, each potentiometer including an impedance member and a rotatable contact member associated therewith, a conductive connection between each terminal of the control potentiometer impedance and a corresponding terminal of the rebalancing potentiometer impedance, each said connection including a fixed impedance element having an impedance equal to more than twice that of one of said impedance members, means responsive to unbalance of said network to drive said rebalanclng potentiometer slider, and means efiective upon relative movement of said control potentiometer contact directly from one terminal of its associated. impedance to the opposite terminal to shunt a portion of said network including one of said fixed impedance elements, thereby unbalancing said network in a sense to cause said driving means to operate said rebalancing potentiometer slider directly from its corresponding one impedance terminal to its corresponding opposite impedance terminal.

2, In an electrical follow-up system, in combination, a continuously rotatable main controller and a continuously rotatable follow-up controller, each controller including impedance means adjustable in a series of substantially equal finite steps by operation of said controller, a controlling member operatively connected to said main controller through a motion reducing gear, a driven member operatively connected to said follow-up controller through a motion reducing gear, a single electrical circuit including both said impedance means, and means responsive to an electrical condition of said single circuit for controlling the operation of said follow-up controller and said driven member.

3. In a remote positioning system, in combination, a transmitter unit and a receiver unit, each The gear ratio between shafts 232 and I06,

unit comprising electrically cooperating relatively rotatable contact and impedance devices, electrical connections interconnecting said transmitter and receiver to form a bridge circuit having a pair of transmitter impedance terminals and a pair of receiver impedance terminals, electrical means responsive to an unbalance condition of said bridge circuit variable in sense in accordance with the direction of relative movement 0! said transmitter devices for controlling relative movement of said receiver devices to rebalance said bridge, compensating impedance means, connections between said compensating impedance means and said bridge circuit, and means operative as an incident to relative movement of said transmitter contact device directly from one of its impedance terminals to the other for changing the connections between said compensating impedance means and said bridge circuit to effectively unbalance said bridge circuit in a direction to cause corresponding relative movement 0! said receiver contact device directly from one of its impedance terminals to its other impedance terminal.

4. In a remote positioning system, in combination, a transmitter unit and a receiver unit, each unit comprising electrically cooperating relatively rotatable contact and impedance devices/electrical connections interconnecting said transmitter and receiver to form a bridge circuit having a pair of transmitter impedance terminals and a pair of receiver impedance terminals, electrical means responsive to an unbalance condition of said bridge circuit variable in sense in accordance with the direction of relative movement of said transmitter devices for controlling relative movement of said receiver devices to rebalance said bridge, variable compensating impedance means, connections between said compensating imped ance means and said bridge circuit, means operative as an incident to relative movement or said transmitter contact device directly from one of its impedance terminals to the other for changing the connections between said compensating impedance means and said bridge circuit to etfectively unbalance said bridge circuit in a direction to cause corresponding relative movement of said receiver contact device directly from one of its impedance terminals to its other impedance terminal, and means operable in accordance with the displacement between the angular positions oi the rotatable devices of said transmitter and receiver for varying said variable compensating impedance.

5. A remote position control system, including in combination, a transmitter unit and a receiver unit, each unit comprising a stationary member and a movable member continuously rotatable about a center on said stationary member, one

,of said members in each unit comprising elongated impedance means, the other of said members in each unit comprising a slider for engagmg said impedance, a normally balanced electrical network including said impedance and slidassess:

allel with a portion of said network, thereby unbalancing said network in a sense to cause said unbalance responsive means to drive the receiver slider directly between the terminals of its associated impedance means, and means eiiective when the displacement between the angular positions of said movable members exceeds a predetermined value to shunt a portion of said network so as to additionally unbalance said network.

6. Control apparatus comprising in combination, a main controller and a follow-up controller; each controller comprising first and second variable electrical devices, means i'or simultaneously varying the devices oi the main controller at diflerent rates, a first electrical network including both said first devices, a second electrical network including both said second devices, means responsive to unbalance of one of said networks for varying the devices of the follow-up controller at diflerent rates so as to rebalance said networks simultaneously, and means'responsive to a predetermined degree of unbalance in the other oi said networks for additionally unbalancing said one network.

7. In an electrical follow-up system, a main controller and a follow-up controller, each conthe opposite terminal of said impedance, a con.

trolling member operatively connected to, the retatable contact arm of said main controller, a

- driven member operatively connected to the rotatable contact arm of said follow-up controller, relay means operative in accordance with the relative electrical potentials of said main and follow-up contact arms for controlling the operation 0! said driven member and being elective to cause the rotatable arm of said follow-uncontroller' to assume a position corresponding to the position of the arm 0! said main controller, an

electrical network including said impedance members, and circuit means including switch means operable by said contact arms and completed when said contact arms are in electrical connection with opposite terminals of their corresponding impedance members to introduce an unbalance eflect in said network such that said relaymeansisoperatedinasensetocausethe contact arm 01' the follow-up controller to move through the relatively short distance between one terminal oi the follow-up impedance and the opposite terminal.

8. In an electrical follow-up system, a main controller and a follow-up controller, each controllerincluding an impedance member tapped at a plurality of spaced points, a plurality of contact segments, one segment being connected to each or said tape, and a rotatable contact arm adapted to selectively engage said segments so astovarythepotential of said contactarmin a series 01 substantially equal finite steps, said segments being spaced circumierentially about the axis otrotatlon or said am. so that said am may be moved through a relatively short distance from electrical connection with one terminal of said impedance into electrical connection with the opposite terminal 01' said impedance, a con trolling member operatively connected to the rotatable contact arm of said main controller, a driven member operatively connected to the rotatable contact arm of said follow-up controller, relay means operative in accordance with the relative electrical potentials of said main and follow-up contact arms for controlling the operation of said driven member and being efiective to cause the rotatable arm of said follow-up controller to assume a position corresponding to the position of the arm of said main controller, an electrical network including said impedance members, compensating impedance means connectable in said network and effective when so connected to make the electrical potential between one terminal of the main impedance and the opposite terminal of the follow-up impedance substantially equal to one of said steps, and circuit means including said compensating impedance means, and switch means operable by said contact arms and completed when said contact arms are in electrical connection with opposite terminals of their corresponding impedance members to connect said compensating impedance means in said network so that said relay means is operated in a sense to cause the contact arm 01' the follow-up controller to move through the relatively short distance between one terminal of the follow-up impedance and the opposite terminal.

9. A normally balanced electrical network, comprising in combination, control means for producing in said network an unbalance eflect variable within predetermined limits, means responsive to said unbalance effect for rebaiancing said network, and means eflective upon operation of said control means so as to produce a first unbalance effect greater than a predetermined value for shunting a part 01' said network so as to produce therein a second unbalance efl'ect opposite to and greater than said first eiiect.

10. A normally balanced electrical network, comprising in combination, variable control impedance means for unbalancing said network, variable follow-up impedance means for rebalancing said network, means responsive .to unbalancerci said network for operating said follow-up impedance, variable compensating impedance means normally disconnected from said network, means eil'ective in response to .a predetermined degree of unbalance of said network for connecting said compensating impedance means in parallel with a portion of said network, and means i'orvarying said compensating impedance means in accordance with the amount of unbalance in excess of said predetermined degree.

11. A remote position control system, including a in combination, a transmitter unit and'areceiver unit, each unit comprising a-stationary member and a movable member continuously rotatable about a center on said stationary member, each stationary member comprising an elongated impedance element having terminals at-each end and tap connections at a plurality oi spaced I points thereon, a row oi arcuate contact segments to an angular position corresponding to that of said transmitter movable member, and means including additional contacts engageable with said sliders and effective when said sliders engage segments on opposite sides of the space between said two adjacent terminal segments to unbalance said bridge in a sense to cause said driving means to operate said receiver slider across said space.

12. In an electrical bridge circuit, in combination, a control potentiometer and a rebalancing potentiometer, each of said potentiometers having relatively movable portions, one of said portions including an elongated impedance member having a longitudinally extending contact area and first and second electrical terminals, the other of said portions including contact means arranged for effectively unidirectionally cyclic motion in any selected one of a plurality of opposite senses, said unidirectional motion including an interval of movement of said means from said first terminal to said second terminal along said member and a second interval of movement of said means from said second terminalto said first'terminal without intervening contact with said member, conductive connections between said first and second terminals oi said control impedance member and said first and second terminals respectively of said rebalancing impedance member, each of said connections including a, fixed impedance element having more than twice as much impedance as one of said impedance members, means oppositely responding to opposite unbalances of said network for driving said rebalancing slider in opposite directions with respect to said rebalancing impedance member, whereby movement oi. said control slider in a selected sense may: result in movement of said rebalancing slider also in said selected sense, and means eiiective upon movement 01' said control slider continuously in a selected sense'through said second portion of said cycle to shunt a portion of said network including one of said fixed impedance elements, whereby to independently establish such unbalance of said network as to maintain motion of saigfrebalancing slider in said selected sense.

13. An electrical bridge circuit comprising first and second variable impedances, means for operating said first impedance to unbalance said circuit, means responsive to unbalance oi said circuit for operating said second impedance to rebalance said circuit, whereby continued operation of said first impedance in a selected sense may cause continued operation of said second impedance in'a predetermined sense. said impedances having electrical discontinuities, compensating impedancemeans, and means associating said compensating impedance means and said circuit whereby to extend operation oi said second impedance means in said predetermined sense during operation 01' said first impedance means through one 0! said discontinuities.

SEGI'RIED G. ww 

